The invention is directed to a method of recycling waste paper in a deinking process with flotation and at least one additional downstream cleaning step. A variety of such deinking processes are known in the art.
An essential component of modern deinking systems is selective flotation, which results in extensive removal of printing inks from the fiber suspension formed during flotation with minimal loss of fibers. However, this step yields foam that is highly contaminated with printing inks and also has a considerable reusable fiber content. Workup of this foam is an important problem of deinking plants.
According to the processes described in German Patent Nos. 15 17 227 (Jul. 17, 1975) and 26 10 581 (Aug. 11, 1977), such foam is purified in a centrifuge to produce relatively clear water and sludge. The sludge is discarded and the clear water, although still highly contaminated, is purified in a clarification system in the paper plant. Naturally, this leads to a great burden on the clarification system. The loss of fibers can be minimized only by performing so-to-speak fractionation (i.e. purification) of the foam of preceding flotation stages in secondary cells. However, this does not avoid a heavy burden on the clarification system.